There are several reasons why you would like to charge your N900 battery in a N810. Maybe you have a spare battery that you want to charge while using the other battery in your N900, or maybe your N900 refuses to charge the battery by itself. Please note that even though it works for me, the standard disclaimer applies - it's all your fault if this destroys your devices and/or batteries :)
Now, let's have a look at the pinouts and connectors of both batteries:
So, the connectors seem to have the same size, even though the batteries do not. Trying to put the N900 battery into the N810 is not straightforward, as there is a white piece of plastic that prevents the battery from getting close to the connectors. The solution is to simply use the N810 stylus as a lever to push the battery towards the connectors:
After the stylus and battery are fixed, simply connect the N810 to the power adapter, and it should start charging. Thanks to Quim for this helpful hint :)